The risk of graft-versus-host-disease (GVHD) bars a high proportion of leukemia patients from the benefits of a graft-versus-leukemia (GVL) effect. The goal of this project is to determine, in a murine model, whether GVHD can be controlled by using "suicidal lymphocytes," and if so, to optimize their use in the transplant setting. The immunologically mediated graft-versus-leukemia (GVL) effect noted after allogeneic bone marrow transplantation (allo BMT) is a powerful anti-leukemic, but its use as a therapeutic modality is limited by the development of GVHD. The risk of GVHD rises with patient age and donor-recipient mismatch. Current GVHD therapy is immunosuppressive: it treats the symptoms rather than eliminating the T cells that cause GVHD. Consequently many patients cannot benefit from a GVL effect. Controlling the fate of transplanted lymphocytes after infusion may permit limitation of severe GVHD while maintaining a GVL effect. Such therapy may improve the success rate of allogeneic transplantation; it may eliminate or reduce the barriers that prevent most leukemia patients from benefiting from the GVL effect. To create the ability to eliminate lymphocytes after infusion we propose to use a retrovirus to introduce a suicide gene, herpes simplex virus- thymidine kinase (HSV-TK), to sensitize the cells to ganciclovir (GCV). Normal human cells are insensitive to this drug. The suicide gene-bearing lymphocytes are then selected and expanded ex vivo and introduced into a patient to promote a GVL effect. If GVHD develops, it may be inhibited,a arrested, or terminated by administration of GCV. The proposed studies will determine the efficacy of these transduced lymphocytes in established murine models and investigate means of optimizing their use. Specific Aim 1. Produce suicidal lymphocytes and test their function in vitro. Demonstrate infection, selection by FACS sorting, and expansion of murine lymphocytes transduced by a retrovirus containing the low-affinity nerve growth factor receptor and HSV-TK genes (LNGFR-TK). Demonstrate in vitro sensitivity to ganciclovir and in vitro alloimmune function of LNGFR-TK-infected lymphocytes. Specific Aim 2. Demonstrate the ability of LNGFR-TK-infected lymphocytes to cause GVHD in mice and the ability to eradicate GVHD by in vivo administration of GCV. Determine the natural survival of infused lymphocytes. Optimize cell dose and timing of ganciclovir administration. Specific Aim 3. Demonstrate that infusion of LNGFR-TK-infected lymphocytes into leukemia-bearing mice generates a GVL effect in a transplant setting. Specific Aim 4. Optimize the anti-leukemic effect of the transplanted lymphocytes by combining strategies for separating GVL from GVHD with the use of the suicidal lymphocyte.